1. Technical Field
The present invention relates to a transparent electrode, a conductive laminate and a conductive layer, and more particularly, to a transparent electrode which includes a conductive layer containing photopolymerizable resin and carbon nanotubes, and to a conductive laminate and a conductive layer.
2. Description of the Related Art
As computers and varieties of home appliances and communication devices are digitized and rapidly advance in performance, the demand for a display which has a large screen and is portable is urgent. In order to achieve a flexible display which is portable and has a large screen, there is a need for a display material which is foldable or rollable like a newspaper.
To this end, an electrode material for a display should be transparent with low resistivity and also should exhibit high strength so as to be mechanically stable when a device is bent or folded. Furthermore, this material should have a coefficient of thermal expansion similar to a coefficient of thermal expansion of a plastic substrate so as to prevent a short circuit from occurring or surface resistivity from greatly changing even when a device is overheated or is at high temperature.
Because a flexible display enables the fabrication of a display in predetermined form, it may be applied not only to portable display devices but also to clothing where it is able to change color or pattern, trademarks of clothing, signboards, price signs of display stands, and large electric lighting systems.
In this regard, a transparent conductive thin film is widely used for devices such as image sensors, solar cells, various displays (PDP, LCD, flexible), etc., which need both transmission of light and conductivity.
Although thorough research into indium tin oxide (ITO) for transparent electrodes for flexible displays has been conducted, the formation of a thin ITO film typically needs a process in a vacuum, which undesirably consumes high process costs. As well, in the case where a flexible display device is bent or folded, the lifespan thereof may be shortened because the thin film breaks.
In order to solve these problems, there has been developed a transparent electrode (Korean Unexamined Patent Publication No. 10-2005-001589) having a visible light transmittance of 80% or more and a surface resistivity of 100 Ω/sq or less by increasing conductivity while minimizing scattering of light in the visible light range, in which carbon nanotubes are chemically bound to a polymer and then formed into a film, or a conductive polymer layer is coated with purified carbon nanotubes or carbon nanotubes chemically bound to a polymer so that the carbon nanotubes are dispersed in nano scale inside or on the coating layer, and nanoparticles of metal such as gold or silver are mixed. Specifically, this transparent electrode was manufactured by reacting a dispersion solution of carbon nanotubes with polyethyleneterephthalate, thus preparing a high-concentration carbon nanotube-polymer copolymer solution, which is then applied on a polyester film and dried.
However, in the case where the above transparent electrode is used at high temperature, polymer deformation may occur and it is difficult to form the pattern of the electrode.
In addition, research into use of a conductive polymer which is an organic material as a material for a transparent electrode is being conducted. In the case of an electrode manufactured from a conductive polymer, conventional diverse polymer coating methods may be utilized, thus remarkably reducing the process cost and work. Specifically, in the fabrication of a flexible display or electric lighting system, a transparent electrode made of a conductive polymer such as polyacetylene, polypyrrole, polyaniline or polythiophene is advantageous in terms of process and is more flexible and breaks less, compared to transparent ITO electrodes. Accordingly, when an electrode which is very flexible is needed, in particular, when a touch screen is manufactured, the lifespan of the device may be advantageously prolonged. However, the conductive properties of an organic electrode made of conductive polymer are generally increased in proportion to thickness of the electrode. Furthermore, because the conductive polymer absorbs light in the visible light range, it should be applied thinly to increase transmittance in order to be adapted for a display. In the case where transmittance is increased in the visible light range in this way, it is difficult to satisfy surface resistivity required in application fields of transparent electrodes. In particular, in the case of using polythiophene (Baytron P, available from Bayer) in which conductive polymer nanoparticles are dispersed in water in order to increase processability, even when conductivity and coatability are improved using a solvent mixture, the application of such a polymer to a thickness of 50 nm on a substrate using spin coating makes it difficult to obtain a surface resistivity of 1 kΩ/sq or less.
Also, a conventional organic electrode material using carbon nanotubes is mainly provided in the form of a composite in which carbon nanotubes are simply mixed with a conductive polymer. As such, the carbon nanotubes aggregate extremely in a conductive polymer matrix due to strong Van der Waals force. Because of such aggregation of carbon nanotubes, it is difficult to form an electrode in which carbon nanotubes are uniformly dispersed despite superior conductive properties of carbon nanotubes. Moreover, even when carbon nanotubes are mixed in an amount of 1˜10 wt % with the polymer, this mixture should be applied thickly in order to obtain satisfactory conductivity. The composite of carbon nanotubes and polymer which is applied thickly considerably reduces the transparency of electrode attributable to the carbon nanotubes aggregated in micro scale, and is inappropriate for use in a Hence, an organic transparent electrode material having high transparency and low surface resistivity even when carbon nanotubes are used in a small amount is required.